Planet-Moon Size Ratio

Planet-Moon Size Ratio - Earth and Moon

What is the optimum size for a planet to form a just-right moon? According to a new study, the short answer to that question is, “Just about the size of planet Earth.” Is that another coincidence or a matter of design? First, let’s consider the planet-moon size ratio.

The radius of the Moon is a little more than one-fourth of Earth’s radius. That four-to-one ratio is unlike any other planet-moon size ratio in our solar system. The solar system’s largest moon is Ganymede which orbits Jupiter. It is one-third larger than our Moon, but Jupiter is more than 11 times the diameter of Earth. The essential factor is that our Moon is larger in relation to the size of the planet it orbits than any other moon in the solar system.

Scientists believe that our Moon was formed when a planet the size of Mars collided with the early Earth. The heat caused by the collision vaporized the material, which eventually congealed to form the Moon and planet Earth. (See Genesis 1:1-2.) The angle of that collision was precisely fine-tuned to result in a planet and moon with the exact size ratio for the Moon to control Earth’s tilt, rotation speed, and ocean tides, as well as perfectly covering the Sun during a total solar eclipse.

To suggest that the creation of a planet perfect for advanced life could have been an accident requires that you have a lot of faith in chance. But a new study by researchers from the University of Rochester in New York adds even more evidence for design. They found that a planet more than 1.3 to 1.6 times Earth’s size could not form a moon that would have the “life-enabling effects” that our Moon has.

Computer modeling shows that if larger planets collided, the energy of the impact would vaporize the material, but a stabilizing moon could not form. The vaporized material would cause drag on the planet, slowing it down so much that gravity would cause any material that congealed to crash into the planet. Not only is the planet-moon size ratio essential, but so also is the size of the colliding bodies.

Numerous factors beyond the planet-moon size ratio must be just right for a functional, life-supporting planet to form. Our planet meets every one of those requirements, and so far, astronomers have not found any other planet that does. The Bible does not tell us whether God chose to create life on another suitable planet anywhere in the universe. However, scientists will never stop looking for it. In the meantime, they keep finding factors that show evidence of intelligent design everywhere in the universe.

— Roland Earnst © 2022

References: and Nature Communications